A holographic projection system in the sense of this invention holographically represents preferably moving three-dimensional scenes in real-time with the help of video means and comprises discretely controllable spatial light modulator means, which are encoded with a sequence of video holograms by a hologram processor such that they spatially modulate waves of light which are capable of generating interference with holographic information. The modulated light waves reconstruct object light points in a reconstruction space outside the projection system, in front of the eyes of observers, through local interferences, where said object light points optically reconstruct the desired three-dimensional scene. Reconstructed object light waves which represent the entirety of all object light points propagate in a directed manner towards the eye positions of at least one observer, so that one or multiple observers can watch those object light points in the form of the scene. This means that in contrast to a stereoscopic representation, a holographic representation realises a substitution of the object.
In order to achieve a satisfying quality of holographic representations, the observers should also be able to watch a reconstruction in a sufficiently large range of vision. Consequently, the reconstruction space must be as large as possible, and the size of a holographically reconstructed scene should be at least 50 cm in diagonal, similar to TV and video representations.
A known problem in a reconstruction system is an undisturbed propagation of the required light waves prior to generating interference. In order to reconstruct the object light points at the correct position in space, and with the correct light point values, at least a part of the interfering light waves must arrive simultaneously at all the positions at which object light points are to be reconstructed through interference. This means that spatial coherence is required among as many as possible of the interfering light waves at each desired object light point.
Moreover, the wave lengths of the light waves which contribute to an interference point must not exhibit any uncontrolled path length differences among one another as caused by controllable optical means.
In the description below, the term ‘optical axis’ denotes a straight line which coincides with the axis of symmetry of a reflecting or refracting optical element. Spatial light modulator means, which have been encoded by a hologram processor with holographic information of a three-dimensional scene, represent a ‘video hologram’. The interaction of a video hologram which is illuminated with coherent light with imaging means causes a ‘modulated wave’ to be generated. The modulated wave is a three-dimensional light distribution, which propagates in the form of a Fourier transform of the video hologram towards the eye position, thus reconstructing the scene by way of interference. The imaging means define a ‘direction of propagation’ of the modulated wave. This direction of propagation can be modified by ‘optical wave tracking means’. If optical elements are disposed on the way to or if their effective direction is towards the video hologram, they will be referred to as ‘hologram-side’, and if they are disposed on the way to or if their effective direction is towards an eye position of an observer eye, they will be referred to as ‘observer-side’. A ‘visibility region’ describes a space which is disposed on the observer side at an eye position as the exit pupil of the system, and in which at least one observer eye must be situated for observing a holographically reconstructed scene. If, as is the case in the present application, the projection system includes an optical wave tracking means for tracking the modulated wave to the current eye position, the ‘tracking range’ defines the space which embraces all eye positions for which wave tracking is possible. In the technical literature on the subject, such a projection system is also known as a projection system with eye tracking device.
In the international publication WO 2004/044659, titled “Video hologram and device for reconstructing video holograms”, the applicant of this invention has already disclosed a holographic reconstruction system which describes one possibility of tracking holographic reconstructions.
In that reconstruction system, a wave is modulated with holographic information by spatial light modulator means. The modulated wave reconstructs by way of interference the three-dimensional scene in a virtual reconstruction space outside the holographic system, said reconstruction space being positioned in front of one or both eyes of one or multiple observers. For this, the modulated wave exits the reconstruction system at its observer side through the spatial light modulator means. In order to achieve a wide viewing range, the surface area of the exit pupil of the system should be as large as possible. In order to efficiently utilise the resolution of the light modulator means, focussing means can then again be used to reduce the size of the reconstruction space near the eye to the dimensions of an eye pupil, so that the reconstruction space preferably has the shape of a frustum with an apex angle which is as large as possible, in order to be able to show large objects of a three-dimensional scene in their entirety as the distance between the observer to the reconstruction increases. A visibility region, in which at least one eye of an observer must be positioned for observing the reconstruction, begins at an eye position at the observer-side end of the reconstruction space, in the Fourier plane of the focussing optical system. The illumination device of the spatial light modulator means is imaged in this region, which also used to be referred to as the observer window in many of our previous applications.
The reconstruction space, which is frustum-shaped due to the focussing, causes problems with the visibility of the three-dimensional reconstruction, if the observer eye is not fully situated inside the visibility region. Already a slight lateral movement of the observer may cause effects such as disappearance of visibility, vignetting or distortion of the spatial frequency spectrum. Moreover, the borders of the reconstruction space are difficult to find for an observer whose eyes are situated outside the visibility region. This is why the exit direction of the wave is preferably adapted together with the virtual reconstruction space to the new eye position if an observer moves. To achieve this, the holographic reconstruction system can displace the entire illumination device of the light modulator means or individual parts thereof.
Document “Eye-Position Tracking Type Electro-Holographic Display Using Liquid Crystal Device”, N. Fukay et al., published in Asia Display 1995, pp. 963-964, XP002940561, describes another optical tracking means for tracking holographic reconstructions. Two small spatial light modulators each generate in a spatially multiplexed process a modulated wave, in order to provide a reconstruction for each eye position of an observer. A vertically and horizontally turnable tracking mirror is disposed in the centre of the reconstruction, i.e. outside the holographic display, or more specifically between the display panel and the observer. The system thus generates in parallel two small observer zones, which can be tracked on a truncated circular path around the reconstruction, so as to follow the lateral movements of an observer. Besides the external position of that tracking mirror, another disadvantage is that the observer's viewing direction is crosswise due to the deflection from the mirror, and that it is substantially limited to the side segments along the circular path. The reconstruction system compensates longitudinal movements of the eye position by way of displacing the illumination device of the light modulator, as in the former solution.
The international publication WO 1999/06856, titled “Microscope with adaptive optics”, discloses an adaptively controllable optical system. A wave front modulator for modifying a light wave such as to move the focus within the object space without changing the axial distance between object and objective lens is disposed between eyepiece lens and objective lens in the optical observation and illumination path of a microscope. The wave front modulator realises a phase modulation and thereby deforms the image wave front spherically in the pupil plane of the objective lens, or in a plane which is equivalent to the pupil plane. Further, the optical system is able to correct the wave front which is curved due to the effects of the objective lens by way of accordingly locally adapted modulator adjustments.
The optical system for wave front modulation can be of a reflective type, for example using electrically controlled deformable mirrors, or of a transmissive type, for example using an LCD panel. The optical system can also comprise discretely movable segments, which are controllable so as to compensate local aberrations in the wave front. The focus location in the object is axially displaced by way of spherical modification of the wave front, and the wave front is tilted due to a lateral displacement. The adaptive optical system must be controllable in segments in order to be able to correct angle-specific aberrations. Alternatively, two independent modulators are used which are disposed in different pupil planes. All manipulations are performed in a pupil plane of the optical path.
The international publication WO 2006/119760 filed by the applicant, titled “Projection device and method for the holographic reconstruction of scenes”, describes a holographic projection system in which a plane light wave LW illuminates the entire surface of a spatial light modulator with light which is capable of generating interference. A hologram processor HP dynamically encodes the light modulator with holographic information of a desired three-dimensional scene. The encoded modulator thus represents a dynamic video hologram. The light modulator can work in a transmissive mode, i.e. modulate a wave which is capable of generating interference as it passes through the modulator, or it can serve as a controllable reflector.
Because knowledge of the functional principle of a projection system is essential for understanding the present invention, an exemplary projection system will be described now with reference to FIG. 1. However, the inventive idea can also be realised with the help of other projection systems.
An optical projection system L in a holographic unit HU images a video hologram which is encoded on the spatial light modulator SLM in an enlarged manner into an image plane which coincides with a focussing display screen S. A spatial frequency spectrum of the video hologram is thereby formed in the image-side focal plane of the optical projection system L, the Fourier plane FTL. Because of their matrix arrangement, the modulator cells modulate the wave spatially and equidistantly. As a result, multiple diffraction orders are created in the Fourier plane FTL, which lie at different positions in a periodic spatial sequence. The focussing display screen S would image all diffraction orders into its focal plane, and an observer would see them with an eye outside the visibility region, i.e. the other eye that is not provided with the content of the video hologram. A spatial frequency filter AP disposed in the Fourier plane FTL prevents this as it selects one diffraction order. The focussing display screen S thus only images the desired diffraction order of the modulated wave into its focal plane FL in front of an eye position PE0. An observer can watch the reconstructed three-dimensional scene 3DS behind the eye position PE0.
In the example shown in FIG. 1a, the display screen S is a lens. As explained above, the diameter of the display screen should be very large compared with the optical projection system L. The display screen S is therefore preferably a concave mirror.
The video holograms are encoded such that the reconstruction is only performed when the enlarged and focussed wave has left the system through the display screen S.
However, the reconstruction 3DS is fixed in the reconstruction space in this system too, so that it will only be visible if at least one eye of the observer is situated directly in the visibility region behind the eye position PE0, which is not physically visible. Unlimited mobility in front of the system without loss or restriction of visibility of the reconstruction 3DS will again only be possible if an additional wave tracking means is used.
If the reconstructed scene is to be visible without any restrictions when the observer moves, a position controller CU must track the optical path of the entire modulated wave with the help of optical wave tracking means WFD to the respective observer eye such that the end of the reconstruction space is always close to the respective observer eye PE1, as shown in FIG. 1b. For this, the exemplary projection system comprises an eye finder, known as such, which detects the current positions of the observer eyes and which controls with the help of the position controller CU the optical path of the modulated wave such that the latter is directed at the current eye position PE1. In a system which provides a specific wave for each observer eye, the desired eye position is the position behind which the observer eye that corresponds with the currently encoded video hologram is situated. The video hologram must not be visible at the other eye position.
Because the enlarged modulated wave always only exits the reconstruction system through a limited section of the display screen S, due to the inclination of the optical axis towards an eye position PE1, the display screen S has to be much larger, as can be seen in FIG. 1b. A large section A0 of the display screen S, which varies according to the moving eye position, then always remains unused. Such a solution would be rather difficult and costly and not very convenient in practical use.
It becomes clear from the above explanations that the visibility region in the sense of the present patent application is an image of the filtered spatial frequency spectrum of the video holograms in front of at least one observer eye. The size of this region depends on both the distance between individual modulator cells, i.e. the pitch of the light modulator, and on the focal length of the entire optical system and the optical path length of the light used for reconstruction. The latter two relations are of importance in the context of the present invention. They have the effect that the properties of the desired reconstruction, such as visibility and size, depend on all optical parameters which affect the focal length of the entire optical system and the path length of the modulated wave, in particular in the form of distortion of parts of the wave.
Because the optical wave tracking means in the present projection system are disposed inside the system, the direction of propagation of the modulated wave is not identical to the screen axis for various angles of incidence, which depend on the eye position PE. Depending on the desired working range of wave tracking, this deviation may be in the range of about ±5 degrees around the screen normal, i.e. far beyond the aberration-free region, also known as the Gaussian region. This means that a projection system with optical tracking of the modulated wave greatly departs from the conditions required for perfect imaging of a spatially expanding modulated wave.
A changing direction of propagation of the modulated wave when exiting the optical system has the effect that the display screen biases the spatial structure of the exiting wave, also including aberration portions which depend on the actual direction of propagation. An optically widened wave which impinges on a large focussing display screen is particularly sensitive to aberrations, such as spherical aberration, coma, field curvature, astigmatism and distortion. Aberrations which depend mainly on the field size, i.e. field aberrations, fluctuating portions of which interfere if the eye positions are extremely distant, are particularly disturbing. Depending on the eye position, aberrations of various strengths may also appear in vertical and horizontal direction. Because according to the present holographic reconstruction principle with a focussing display screen the reconstruction can only be realised with an image of the diffraction order of the video holograms which has been selected by the spatial frequency filter, system-specific aberrations of the wave prior to the reconstruction may cause substantial damage as the angle of incidence increases.
Because the described projection system also takes advantage of a special encoding of the light modulator means, which prevents diffraction orders from overlapping with the help of realisable spatial light modulators, field aberrations, hitherto unknown in conventional projection systems, also occur when reconstructing certain object light points of a scene. In this particular case, the hologram processor only encodes each hologram point on the light modulator means in a limited region of the hologram. As a result, object points which are encoded in marginal hologram regions, are affected differently by aberrations than those object light points which are encoded near the axis. The aberrations and vignetting thereby occurring depend greatly on the eye position at which the modulated wave is to be directed. As a consequence, certain object points are not at all reconstructed or in a wrong spatial depth, or individual object points do not appear within the visibility region. This is why measures must be taken to ensure that light waves from the marginal regions of the display screen reach the visibility region for each eye position and that they reconstruct the object light points in the correct spatial depth. More details on the encoding of a reconstruction system with a focussing display screen have been disclosed by the applicant in the international publication WO 2006/119920.
Also if colour video holograms are reconstructed, light waves of different length cause in the lenses inside the system a dependence of the refractive index on the wavelength, also known as dispersion. This dispersion leads to chromatic aberrations of the primary colours required for colour synthesis of the video hologram, which are perceived by an observer mainly in the form of different sizes and positions of the visibility region.
A projection system including wave tracking with at least one movable mirror in front of the display screen thus has the effect of a more or less distinct position-specific change in the geometry of the exiting wave with each shift in direction of the wave which results from a position change of an observer on the one hand, and from time-division multiplexed switching between the positions of different eyes on the other. For certain exposed eye positions, this change in geometry can be so grave that a satisfactory reconstruction of the scene becomes impossible. Means for correcting the wave form are thus desired which realise a specific correction of the wave form for each eye position of an observer within a desired tracking range and for each primary colour so that the wave has constant geometric and optical properties. In the present case, the optical wave correction must be capable of being switched between different correction settings at very high speed, in particular in a projection system which switches between the positions of different eyes in a time-division multiplexed process.
Because in a desired large tracking range, the cumulative aberrations can vary greatly among individual eye positions, caused by the interaction of many different interfering aberrations, aberration correction with the help of fixed optical means is not possible.